Fibre oscillators such as erbium or ytterbium doped fibre lasers as well as Raman amplifiers are reliable devices which are used in many different technology fields such as telecommunications, optical imaging, healthcare and metrology. Much effort has been dedicated to increasing the peak power of the pulses delivered by fibre based lasers as their stability and compact size make them an attractive alternative to bulky solid-state lasers. A particular limitation of pulsed fibre based lasers is the nonlinear phase limit leading to a wave breaking effect leading to degradation of the pulse shape. Fibre lasers also require dispersion management to allow group velocity and self-phase modulation induced chirp to interplay. Each factor limits the potential output power.
A type of waveform known as a parabolic pulse has been developed to overcome the difficulties of nonlinearity and enable high energy fibre based lasers to be constructed. A parabolic pulse retains its parabolic temporal profile during propagation in an optical fibre, also known as self similar propagation. A parabolic pulse can experience very large nonlinearity without suffering from the wave-breaking effect. The temporal and spectral parabolic intensity profiles and linear positive chirp are conserved during propagation. A propagating parabolic pulse has temporal and spectral widths that increase exponentially along the fibre. These specific aspects of parabolic pulse propagation makes them suitable for high powered applications since the linear chirp of such a pulse enables simple pulse compression to shorten the duration.
A variety of laser architectures have also been proposed to obtain parabolic pulse propagation. Such architectures, however, utilise free-space components to compress or mode-lock a laser. Despite good performance of such cavities, they are not compact or robust. Further, such lasers also suffer from having to couple light in and out of a fibre, leading to stability and inefficiency problems.
In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.
It is an object of the present invention to provide a laser that produces pulses to overcome or at least ameliorate some of the abovementioned disadvantages or which at least provides the public with a useful choice. Other objects of the invention may become apparent from the following description which is given by way of example only.